1. Field of Invention
This invention pertains generally to the field of multicrystalline silicon solar cells and manufacturing techniques directed to yield improvements in optical performance of silicon solar cells. More specifically, the invention pertains to use of reactive ion etching (RIE) in the presence of metal catalysts to texture the surface of silicon solar cell material. The techniques described can controllably generate randomly distributed surface features optimally having dimensions comparable, or smaller than solar spectrum wavelengths.
2. Description of the Related Art
Multicrystalline silicon (mc-Si) is a frequently-used semiconductor substrate in the manufacture of silicon solar cells. Single crystal silicon (c-Si) is also used in manufacture of silicon solar cells, but it has certain disadvantages including the fact that it is more expensive than mc-Si. U.S. Pat. No. 6,091,021 describes some aspects of mc-Si solar cell fabrication developed by the present inventors, including use of micromasks (for example, a thin polycrystalline coating such as SiO.sub.2) to aid in etched texturing of mc-Si surfaces. In that patent, techniques are disclosed which demonstrate that minute, randomly distributed micro-craters and pyramidal structures can be formed in the surface of the emitter of a solar cell (or any silicon surface, for that matter). This is accomplished, according to that patent, by performing a deposition of SiO.sub.2, for example, with standard chemical vapor deposition methodologies in common use in the photovoltaics industry. As taught by the '021 patent, the "micro-mask" created by such deposition allows etchant (e.g., SF.sub.6 /O.sub.2) to pass through micro-gaps to selectively etch exposed Si in the gap locations before reaching Si underneath the microcrystals. As a result, a random texturing of the Si surface is achieved. These randomly textured surfaces significantly reduce surface reflection over the usable spectral range, thus, increasing the light absorption in the solar cells. This, in turn, increases the likelihood that additional electron-hole pairs will be generated within the silicon matrix as a result of the photovoltaic effect.
Additional relevant background information can be found in U.S. Pat. No. 6,091,021, which is hereby incorporated by reference, in its entirety. Although the '021 patent represents one approach to silicon (especially, mc-Si) texturing, the need remains for more flexibility in the available texturing technologies compatible with solar cell manufacturing techniques. The present invention offers alternate processes aimed at engineering randomly textured surfaces compatible with existing etching methods and standard solar cell manufacturing processes. The present invention, according to at least one embodiment, also can be adapted to allow optimal random texturing of silicon solar cell material processed at a high throughput rate.